In recent years, attempts for reducing carbon dioxide (CO2) gas, which is a main cause of the global warming phenomenon, have been made worldwide and in all industries.
The lime industry, together with the cement industry, the electric power industry, the steel industry, and the like, is one of the industries which discharge a large amount of CO2 gas. For this reason, the reduction in the amount of CO2 gas discharged in the lime industry greatly contributes to the reduction in the total amount of CO2 gas discharged in Japan.
FIG. 5 shows a common quicklime manufacturing facility in the lime industry. In FIG. 5, reference numeral 1 denotes a rotary kiln (quicklime kiln) for producing quicklime by burning. Note that the rotary kiln 1 is a horizontal kiln, but kilns, such as Meltz kiln and Beckenbach kiln, are known as conventional vertical kilns for producing quicklime by burning.
A preheater 3 for preheating limestone lumps is provided at the kiln inlet part 2 on the left side of the rotary kiln 1 in FIG. 5, and a main burner 5 for heating the inside of the rotary kiln 1 is provided at the kiln outlet part on the right side of the rotary kiln 1 in FIG. 5.
Here, as the preheater 3, for example, a grate preheater, or the like, is used. The grate preheater is configured by connecting a plurality of grates in a ring shape, and is configured such that, as limestone lumps, which are fed to the upstream side of the grate preheater 3 from a feed line 4 and placed on the grates, are successively moved to the downstream side of the preheater 3, the limestone lumps are preheated by the high-temperature exhaust gas fed from the rotary kiln 1 and are introduced into the kiln inlet part 2 of the rotary kiln 1.
On the other hand, the combustion exhaust gas discharged from the rotary kiln 1 is introduced into the preheater 3 and successively sent to the upstream side of the preheater so as to preheat the limestone lumps. Finally, the exhaust gas is exhausted from the exit of the preheater 3 by an exhaust fan 6 via an exhaust line 7.
In the quicklime manufacturing facility configured as described above, quicklime lump is manufactured in such a manner that limestone (CaCO3) lumps are first preheated by the preheater 3, and then burnt in the rotary kiln 1 in high temperature atmosphere at about 1400° C.
In the calcination process, the chemical reaction as represented by the formula: CaCO3→CaO+CO2↑ is caused so as to generate CO2 gas (generation of CO2 gas resulting from the raw material). The concentration of CO2 gas resulting from the raw material is theoretically 100%. Further, fossil fuel is combusted in the main burner 5 in order to maintain the atmosphere in the rotary kiln 1 at a high temperature. As a result, CO2 gas is also generated by the combustion of fossil fuel (generation of CO2 gas resulting from the fuel). Here, much N2 gas in the combustion air is contained in the exhaust gas discharged from the main burner 5, and hence the concentration of CO2 gas contained in the exhaust gas and resulting from the fuel is as low as about 15%.
As a result, the high-concentration CO2 gas resulting from the raw material and the low-concentration CO2 gas resulting from the fuel mixedly exist in the exhaust gas discharged from the rotary kiln 1. Therefore, in spite of the fact that the discharge amount of CO2 is large, there is a problem that the CO2 gas has a concentration of about 30 to 35% and hence is difficult to be recovered.
On the other hand, as the CO2 gas recovery methods which are being developed at present, there are methods based on a fluid recovery system, a membrane separation system, a solid adsorption system, and the like. However, the methods have a problem that the cost for recovering CO2 gas is still very high.
Further, as a method to prevent the global warming due to CO2 discharged from the quicklime manufacturing facility, a method has also been proposed in which CO2 discharged at a low concentration from the discharge source is separately recovered so as to be condensed up to a concentration of about 100%, and is then liquefied so as to be stored in the ground. However, in this method, the cost for separating and recovering CO2 is high, and hence this method is not realized for the same reason as that for the above described methods.
On the other hand, an apparatus for producing and recovering CO2 gas has been proposed in the following Patent Literature 1 as an apparatus for recovering CO2 gas generated in the process of burning limestone as CO2 gas having a high utilization value and a high purity. The apparatus includes a decomposition reaction tower to which limestone is fed, a reheating tower to which quicklime (CaO) is fed as a heat medium and which also heats the quicklime to a temperature not less than the calcination temperature of limestone with a combustion gas, and a connecting pipe which connects the decomposition reaction tower with the reheating tower.
The above-described conventional recovering apparatus is configured such that the quicklime which has been heated in the reheating tower is fed to the decomposition reaction tower through the connecting pipe to form a fluidized bed, such that CO2 gas is produced in the decomposition reaction tower by burning the limestone and also a part of the produced quicklime is discharged, and such that the other part of the quicklime is again sent to the reheating tower through the connecting pipe so as to be reheated in the reheating tower.
In this way, in the above-described apparatus for producing and recovering CO2 gas, the decomposition reaction tower which is a place for conducting the decomposition reaction of the limestone therein is separated from the reheating tower which is a place for generating the amount of heat necessary for the decomposition reaction therein. Thereby, it is possible to prevent the CO2 gas generated through the decomposition reaction of the limestone from being mixed with the combustion exhaust gas generated for heating the heat medium. Accordingly, the apparatus for producing and recovering CO2 gas is considered to be capable of recovering CO2 gas in high concentration from the decomposition reaction tower.